1. Field of the Invention
This invention broadly relates to drying an organic amine chelating agent and more particularly to the volume reduction of an aqueous medium containing the same. In one of its more particular aspects this invention relates to a process for reducing the volume of a low-level radioactive aqueous waste containing an organic amine chelating agent. In another of its more particular aspects, this invention relates to a process for producing a dry, flowable powder from such a waste.
2. Prior Art
Waste management frequently involves the necessity of disposing of large volumes of materials, some of which may be contaminated with hazardous substances. In nuclear power plants, for example, large amounts of radioactive liquid and solid wastes are produced. Low-level radioactive wastes differ from high-level radioactive wastes, which are produced in the reprocessing of nuclear fuels, in that the latter present greater risks of contamination and therefore require disposal techniques which are more stringent than in the case of low-level radioactive wastes. Disposal of radioactive wastes in general cannot be readily accomplished by using conventional waste disposal techniques. Because of the relatively long half-lives of certain radioactive elements, the most widely used disposal techniques are storage, solidification and burial. The expense of so disposing of large volumes of radioactive wastes, however, is constantly rising and approaching levels at which volume reduction becomes not only economically desirable but a necessity.
Many efforts have been directed at reducing the volume of radioactive wastes.
U.S. Pat. No. 3,101,258 describes a heated-wall spray calcination reactor useful for disposing of nuclear reactor waste solutions. In spray calcination reactors of the heated-wall type, however, the temperature gradient from the outside of the reactor inward may result in uneven heating, producing regions of undesired high temperatures and causing non-uniform results.
U.S. Pat. No. 3,922,974 discloses a hot air-fired furnace for incinerating radioactive wastes. The use of this apparatus, however, results in the production of noxious off-gases which require additional processing for removal.
U.S. Pat. No. 4,145,396 describes a process for reducing the volume of organic waste material contaminated with at least one volatile compound-forming radioactive element selected from the group consisting of strontium, cesium, iodine and ruthenium. The selected element is fixed in an inert salt by introducing the organic waste and a source of oxygen into a molten salt bath maintained at an elevated temperature to produce solid and gaseous reaction products. The molten salt bath comprises one or more alkali metal carbonates and may optionally include from 1 to about 25 wt. % of an alkali metal sulfate. Although effective to some extent in reducing the volume or organic wastes, further volume reduction involving the separation of the radioactive materials from the non-radioactive components of the molten salt bath requires a number of additional processing steps.
In U.S. patent application Ser. No. 451,516, filed Dec. 20, 1982 (now U.S. Pat. No. 4,499,833) and assigned to the assignee of the present invention, there is proposed a process for converting radioactive wastes in the form of liquids, solids and slurries into a mixture of a non-radioactive gas and a radioactive inorganic ash. In accordance with that process the radioactive waste is introduced as a finely atomized spray into a zone heated by means of a hot gas to a temperature sufficient to effect the desired conversion, preferably a temperature in the range of about 600.degree. to 850.degree. C. The process is conducted in a spray dryer modified to combust or calcine the waste.
While the foregoing patent application discloses a process which is satisfactory for destroying most radioactive wastes, the high-temperature utilized in the process can produce noxious gases such as NO.sub.x or SO.sub.x, the removal of which necessitates taking additional measures to ensure that any gas ultimately released to the atmosphere is non-polluting. In addition, such high temperatures may cause the volatilization of radionuclides from the radioactive waste and vaporization of some of the constituents of the waste material.
In the nuclear industry various organic amine chelating agents are utilized for cleaning the interior surfaces of the primary coolant loop of the reactor, a typical chelating agent being ethylenediaminetetraacetic acid (EDTA). Such chelating agents are used extensively for cleaning the interior surfaces of the primary coolant loop since they have an affinity for a variety of metal ions. In use, the chelating agent is used in an aqueous medium. Since the acid form of the chelating agent is substantially immiscible in water, it is common practice to add a material to increase its solubility. Typically, the material will be a sodium salt of the chelating agent. After use, the aqueous medium will also contain radioactive isotopes of various metals such as cobalt, manganese, cesium, iron etc.
Heretofore there has been no truly effective way of treating such an aqueous medium. More specifically, the chelating agent contains both a source of oxygen and a source of fuel and has a relatively low decomposition temperature. Thus, treatment at any elevated temperature would result in decomposition and combustion of the chelating agent. Conversely, if treated at a lower temperature to evaporate water and reduce the volume, the resulting residue has a sticky consistency and is difficult to handle or transport. The reason is not known with certainty, but is surmised that perhaps the combination of the chelating agent, metal ions and sodium salt form a highly hydrated complex at temperatures below the decomposition temperature of the chelating agent.
Typically, the aqueous medium containing the chelating agent and metal ion have a very low radioactivity and it would be acceptable to bury the solids content of the aqueous medium in drums in special, set-aside areas where ground water leakage and interaction with other radionuclides are controlled. The complex formed between the chelating agent and the metal ion, however, is water soluble. Thus, the common method for disposal of a spent aqueous medium containing a chelating agent is by solidification in cement. Obviously, this type of disposal technique will generally result in a net increase in volume. Further, the overall cost for such a disposal technique is quite high.
Consequently, there is a need for a process which can be used to reduce the volume of such a radioactive waste without producing noxious off-gases or volatilizing the chelating agent or radionuclides. This need is particularly pronounced in the case of liquid low-level radioactive wastes where large volumes of wastes of relatively low radioactivity compound the problems and costs involved in their transportation and disposal.